Charged particle beams, such as those that are used in focused ion beam or scanning electron microscopes, are typically focused by scanning the beam over a sample with sharp edges.
Other methods for focusing electron beams are known in the art. For example, U.S. Pat. No. 5,483,036, to Giedt, et al., whose disclosure is incorporated herein by reference, describes a method for automatically focusing an electron beam by determining the beam size. The beam is swept over a number of narrow slits, and the current profiles generated as the beam sweeps are used to find the beam size, and to focus the beam to an optimal position.
U.S. Pat. No. 5,726,919, to Azad, et al., whose disclosure is incorporated herein by reference, describes a system for measuring the effective focus of an electron beam. The electron beam generates a temperature profile of an irradiated target, and the temperature profile is measured optically. Beam focus operating parameters are varied until an error between the measured temperature profile and a predicted profile for an effective focus are less than a predetermined value.
U.S. Pat. No. 6,621,082, to Morita, et al., whose disclosure is incorporated herein by reference, describes an automatic focusing system for a scanning electron microscope. The system uses focal positions from an optical microscope and the electron microscope to automatically adjust the focus of the electron microscope.
U.S. Pat. No. 5,614,713, to Kobaru, et al., whose disclosure is incorporated herein by reference, describes a scanning electron microscope wherein a focusing correction signal is produced using a light beam.
U.S. Pat. No. 5,130,540, to Yamada, et al., whose disclosure is incorporated herein by reference, describes a method for automatic focusing of an electron microscope using an auxiliary, coil that is close to the objective lens of the microscope.
U.S. Pat. No. 5,032,725, to Kanda, whose disclosure is incorporated herein by reference, describes a method for focusing a beam of an electron microscope. The disclosure states that an output signal is investigated for one magnification or scanning time of the beam over a range of foci to determine the optimum output value for that one magnification. The disclosure also states that the investigation may be repeated for other magnifications to generate a series of optimum output values, and that the microscope may then be operated at the focus corresponding to the optimum of the series of optimum values.
U.S. Pat. No. 4,933,553, to Tomizawa, et al., whose disclosure is incorporated herein by reference, describes focusing an electron beam of an electron microscope. An optimum exciting current for a focusing lens is determined by measuring a centroid of an area generated by the microscope.
U.S. Patent Application 2006/0049364, to Sender, whose disclosure is incorporated herein by reference, describes a method and system for focusing a charged particle beam.